I've found that when I spend more time riding "in the drops," I automatically use my gluts and hammies more than when I ride slightly more upright. I also agree that if you focus your attention on using those muscles, no matter what your position, you will use them more efficiently. The benefit is more strength in your pedaling, and a more even load on your muscles, which lets you use all of them for a longer time. Or so the theory goes...................

Annie